1. Field of the Invention
This invention relates to spread-spectrum communications, and more particularly to a spread-spectrum signal processing technique through the use of a single phase shift encoded tapped-delay line surface-acoustic-wave correlator (SAWC) to demodulate multiple phase shift keyed (PSK) codes.
2. Description of Related Art
The effects of surface-acoustic waves applied to a piezoelectric material to convert electrical energy to acoustic energy and vice versa for analog signal processing purposes have been known and practiced in the prior art for many years. This process, as applied to phase coded SAW correlators, consists, in its most basic form, of an input transducer and a phase coded tapped-delay line matched to a phase shift encoded carrier. In general, this operation is carried out by applying an electrical signal to a transducer which consists of a sequence of metallised interdigital finger pairs deposited on the surface of a piezoelectric material. The transducer converts this electrical signal to an acoustic wave which propagates down the surface of the substrate to the tapped-delay line. Acoustic energy is converted to electrical energy at the metallised delay-line taps. When the phase encoded wave matches the phase configuration of the delay line taps, the electrical signals are added in phase with each other, and a correlation signal, which provides a signal to noise improvement, is generated and coupled to other electronic circuits through the busses of the tapped-delay line. Multiple correlations may be accomplished by placing separately encoded tapped-delay lines in parallel on the same substrate. Distinct correlation pulses will then occur upon application of matched phase shift encoded signals at the transducer input.
The reciprocal properties of SAW devices allow for this process to take place in reverse, where the tapped-delay line is excited by a phase coded electrical signal, and a correlation signal will occur at the output of the transducer. These reciprocal properties have been discussed for many years in literature and conferences. Certain kinds of devices and signal formats have shown more promise in this area than others. In 1973 J. Burnsweig of Hughes Corp. published a paper detailing the use of linear FM pulse compression matched filters operating in reciprocal manner ("Ranging and Data Transmission Using Digital Encoded FM Chirp Surface Acoustic Wave Filters", IEEE Transactions in Microwave Theory, Vol. MTT-21, pp 272-279, April 1973). This approach involves exciting the long tapped-delay line with the linear FM encoded signal and utilizing the transducers, located a certain distance away from each end of the delay line, as the elements that coherently sum the waveform segments to produce a compressed pulse. The reciprocal approach with the linear FM chirp waveform was utilized to differentiate between a "one" bit and a "zero" bit for satellite ranging/data transmission applications.
While one and zero bit differentiation has been applied toward a number of phase shift keyed (PSK) waveforms, most of these approaches appear to involve some form of acousto-electric convolver and a hybrid network. The simpler approach presented in this invention employs a SAW BPSK matched filter configuration with two transducers located near the ends of a phase coded tapped-delay line. As briefly described above, the two transducers are typically utilized as inputs either to assist in generating the BPSK sequence or serve as the input for the BPSK encoded waveform, and the tapped-delay line serves as the summing network to generate the correlation peak. The reciprocal approach involves the use of the SAW tapped-delay line as the input structure for a one bit code and a reciprocal code representing the zero bit. Coherent summation of the BPSK sequence can be sensed, at a minimum within a chip width from one edge of the tapped-delay line. The summation can be sensed by an appropriate transducer structure that has dimensions corresponding to one chipwidth along with having the correct interdigital finger spacing for the center frequency.